The present invention relates to a system for reproducing digital magnetic recording information recorded on a magnetic recording tape, a magentic recording disk or the magnetic media.
In a conventional digital magnetic recording and reproducing apparatus using an induction type magnetic head, for example, a floppy disk drive (hereinafter referred to as an FDD), the demodulation of written digital information of a detection signal A outputted by a magnetic head 1 as shown in FIG. 1 is effected by amplifying the head detection signal A using a pre-amplifier, eliminating the unnecessary high frequency component using of a low-pass filter 3, the filtered signal passing through a differentiator 4 for differentiation thereof, applying the differentiated signal to a zero crossing detector 5 using a comparator, and detecting a peak point of the head detection signal A as a zero crossing point of the differentiated signal. The zero crossing point theoretically coincides with the position of each bit in a row of clock and data patterns in a modulation system which basically employs a NRZI modulation (MFM modulation, FM modulation, 2-7 conversion, and others), whereby write data is thus demodulated from the detection signal. That is, then the detection signal is demodulated through a read data pulse generator 6.
In the above-described digital magnetic recording and reproducing apparatus, the detection signal A from the magnetic head is applied to the differentiator 4 in order to detect the leading edge of the write data, but peak positions P.sub.1 ' and P.sub.2 ' of peak values of the input signal waveform W of the differentiator 4 results in a peak shift caused by the resolution of the head and medium and the data and clock pattern.
FIG. 2 illustrates one example. Let W.sub.1 be the input signal waveform in the case of only 1P bit of write data, and let W.sub.2 be the input signal waveform in the case of only 2P bit of write data. The input signal waveform W to the differentiator 4 is a composite waveform of W.sub.1 and W.sub.2, and the peak positions P.sub.1 ' and P.sub.2 ' thereof are respectively shifted toward the left or right by .alpha. or .beta. as shown.
This peak shift is varied in its amount and direction according to the write data pattern and recording density. However, if the peak shift is present, the bit spacing of a reproduced signal will be different from the bit spacing of the write data. This reduces the reliability of the digital magnetic recording and reproducing apparatus, and is an impediment to storing the write data written on a floppy disk (hereinafter referred to as an FD) at a high density.
Also, according to the aforementioned conventional method, a depression (which is called a saddle) occurs in the differentiated waveform in the area of low density. Accordingly, a protective circuit has been required prohibit to erroneous detection of the saddle as a data bit by the comparator.